The present invention relates to an electrophoretic positive working photosensitive organic composition, and more particularly, it relates to an electrodeposition composition used in the coating of a metal base plate and forming a thin film thereon. After the exposure and the development of the thin film, it forms a pattern on the base plate which is suitably used in the production of polymeric patterns, especially the production of circuit patterns on printed circuit boards.
The most commonly used method in the production of printed circuit boards is the print and etch method. It is a method for forming a corrosion resisted pattern on a copper base plate which is totally electroplated with copper. The bare copper layer is removed by an etching solution, then a conductor is copied from the corrosion resisted pattern which is removed later by using a stripping solution. In the past, the method used to prepare a corrosion resisted pattern is the screen printing method wherein the transformation of the pattern is not all that definite, i.e. the resolution is low. In order to improve the resolution of the pattern, photoresist is applied so that the copper layer is firstly covered with photoresist and then exposed selectively in accordance with the conducting pattern on a film. The photoresist on the non-conductive part is removed by developer and therefore a corrosion resisted pattern is obtained. This method has a higher resolution than the screen printing method, and thus it has become an important method used in the production of printed circuit boards.
Photoresists used in production of printed circuit boards can be divided into two types, i.e. the liquid type and the dry film type. The coating methods used include for example dip coating, roller coating, spin coating and electrodeposition etc. Since the composition of the liquid photoresist is solubilized by a solvent, after coating, it should be dried before the following step is carried out. Although the compositions of the dry film photoresists are almost the same as the liquid state photoresists, the difference is that the dry film is protected by two upper and lower films a which film materials are polyester and polyethylene, and a copper surface is laminated with the photosensitive layer through the use of thermal compression. Therefore, the exposing step can be proceeded directly without the drying step.
According to the differences in the reacting mechanisms of exposure, photoresists can be divided into positive working and negative working photoresists. The positive working photoresists, wherein the photosensitive compound produces carboxyl groups during the exposure a have a higher higher solubility in an alkaline exposing solution, and thus the pattern is achieved or completed by using this characteristic. For a negative working photoresist, photocuring takes place on the exposed part. Since the cured part is insoluble in an alkaline developing solution, a corrosion resisted pattern is then produced. Negative working photoresists are commonly used in the industry of printed circuit boards.
In the production of multi-layered printed circuit boards, it is hard work to produce a plated through hole, and the common methods used are the pattern plating method and the tenting process. The reliability of the pattern plating method is high but the method is too complicated. On the other hand, the process for tenting is less complicated, but there is a problem with oxygen inhibition inside the hole when using a dry film photoresist so that the degree of curing is not enough. As a result, the photoresist on the tent is easily destroyed by spraying of developing solution, and thus, the reliability of this method is decreased.
An electrophoretic positive working photoresist is uniform in the thickness of the film so that it can cover the whole plated through hole. Since the hole does not require an exposure, the oxygen inhibition problem will not happen. In addition, positive working photoresists are a kind of aqueous emulsion and therefore a lesser amount of organic solvent is used. Additionally the waste is lower when a positive photoresist is used. As a result, it is a kind of potential new material.
Electrodeposition is applied in the coating of photoresist, for example, U.S. Pat. No. 3,738,835 discloses a coating on a metal plate by using a negative working photoresist composition. Furthermore, since the electrophoretic positive working photoresist is advantageous in the coating of a plated through hole, different kinds of photosensitive compositions are disclosed in some patents. For example Japan Laid-Open Patent No. 88-297474 discloses a composition containing a maleic copolymer, Japan Laid-Open Patent No. 89-4672 and No. 89-90270 disclose a composition containing an acrylic copolymer, and U.S. Pat. No. 4,681,923 discloses a composition containing a novolak resin. In the photosensitive groups of the above mentioned patents, photosensitive quinone diazide is grafted on a charge carrying polymer. Quinone diazide reacts with a little amount of water in photoresist after exposure, and then it converts into carboxyl group so that the exposing part is soluble in alkaline developer. For these single component photoresists, the photosensitive groups are immobilized on the side chains of a polymer, which results in a decrease in reactivity and photosensitivity. In addition, a pinhole is formed since the thermal fluidity of a single component photoresist is not good. On the other hand, photosensitive quinone diazide can also be attached or bonded to a small uncharged molecule. For example U.S. Pat. No. 4,673,458 and European Patent No. 0,265,387 disclose the bonding of quinone diazide on 2,4-dihydroxybenzophenone or 2,3,4-trihydroxybenzophenone. Although this method can improve the photosensitivity of the photosensitive groups, the groups are hydrophobic and precipitates are easily formed in emulsion solution such that a problem of the stabilization of electrodeposited photoresist solutions. As a result, the surface of photoresist film is rough and, in turn, reduced the resolution.